Peel and thinly slice your onions. Get a large pot hot. Gloss with oil. Add your onions and a good pinch of salt and pepper. Lower heat. Sizzle for 5 mins, or till tender and a little golden.
Trim the tops and bottoms of your salsify. Peel. Cut into 2cm slices, at a diagonal for prettiness. Rinse them well (they can be sticky). Add to the pot. Grate in the lemon zest and a squeeze of juice. Cook over low heat.
While it cooks, peel and finely chop two garlic cloves. Scrub or peel your carrot. Coarsely grate. Finely dice the celery. Stir into the onion/salsify mix. Sizzle over low heat.
Add your boiling water or stock to the veg. Simmer till the salsify is just tender.
Pick out any larger watercress stalks. Taste your soup. Season as needed. Pile the watercress on top. Take off the heat.
Peel the remaining three garlic cloves. Thinly slice. Heat up a small frying pan. Gloss with oil. Add the garlic and a pinch of salt. Sizzle till just golden.
Bowl up the soup. Spoon the crispy garlic and any oil on top.